In the manufacturing of glass articles, raw materials are converted at high temperature into a homogeneous melt which can be formed into the articles. Sand is the most common ingredient in glassmaking. Other commonly used glass forming materials are soda ash, calcium limestone, dolomitic limestone, aragonite, feldspar, nepheline, litharge, boron-containing compounds, various fining, coloring and oxidizing agents as well as broken glass also known as cullet. In addition to soda ash (Na.sub.2 CO.sub.3), a number of other alkaline or alkaline-earth compounds are used during glass making. Examples include caustic soda or NaOH and various carbonates such as K.sub.2 CO.sub.3, Li.sub.2 CO.sub.3, MgCO.sub.3, CaCO.sub.3, SrCO.sub.3 and BaCO.sub.3.
Melting glass forming materials takes place in a melting unit, for example a melting pot, vessel, tank or furnace. Since the temperatures required to melt glass forming materials are among the highest needed to operate industrial furnaces, special materials are necessary to line the interior surfaces of these glassmelting units.
An additional challenge during glass melting is presented by the corrosive nature of the process itself. Refractory linings are exposed to both physical and chemical attack from molten glass and from some of the vapors generated during the high temperature melting operation. Particularly corrosive are vapors produced from those glassmaking ingredients containing alkaline and alkaline-earth species.
One advance made in glass melting pertains to directly fired furnaces and involves combustion processes where the conventional oxidant, air, is replaced with pure oxygen or with oxygen enriched air. Oxygen-based combustion results in even higher furnace temperatures. For example, the flue gases produced have temperatures generally exceeding 2000.degree. F., typically between 2400.degree. F. and 2800.degree. F.
Not only are refractories lining these furnaces exposed to higher temperatures, but also oxygen-based combustion influences the corrosion effects attributed to compounds containing alkali and alkaline-earth metals. The relatively low gas velocities present in oxygen-fired furnaces slows the mass transfer rate, leading to a reduction in the amount of species volatilized from the glass melt and due to the significant reduction in nitrogen, the partial pressure of all components present in the furnace atmosphere increases and the concentration of vapors containing alkaline or alkaline-earth species can be as much as three to four times higher when compared to concentrations found in conventional air-fired furnaces.
Many existing furnaces employ silica-based refractories. Although materials that can better withstand corrosion are available, such as, for example, zirconia-based refractories, replacing conventional silica linings with more advanced materials imposes significant capital expenses and often requires modifications in the furnace structure, since nobler refractories tend to have a higher density than those containing silica.
There continues to be a need, therefore, to prevent, control or reduce the effects of corrosion observed in glass melting furnaces lined with conventional silica-based refractory materials, especially when such furnaces are operated under oxy-fuel conditions.
Accordingly, it is an object of the invention to provide silica-based refractories having improved corrosion resistance.